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Deep exclusive reactions at Jefferson Lab Hall A
C. Munoz Camacho
To cite this version:
Carlos Munoz Camacho∗
LPC-Clermont, CNRS/IN2P3
E-mail:munoz@jlab.org
Recent results on the Generalized Parton Distribution (GPD) program at Jefferson Lab (JLab) will
be presented. The emphasis will be in the Hall A program aiming at measuring Q2−dependences
of different terms of the Deeply Virtual Compton Scattering (DVCS) cross section. This is a fundamental step before one can extract GPD information from JLab DVCS data. The upcoming
program in Hall A, using both a 6 GeV beam (2010) and a 11 GeV beam (≈ 2015) will also be
described.
XVIII International Workshop on Deep-Inelastic Scattering and Related Subjects April 19 -23, 2010
Convitto della Calza, Firenze, Italy
Deep exclusive reactions at Jefferson Lab Hall A Carlos Munoz Camacho
1. Introduction
Understanding of the fundamental structure of matter requires an understanding of how quarks and gluons are assembled to form the hadrons.
The asymptotic freedom of Quantum Chromodynamics (QCD) makes it possible to use per-turbation theory to treat interactions of quarks and gluons at short distances. In high-energy scat-tering, short-distance and long-distance physics may be separated to leading power in momentum transfer–an approach known as “factorization”. For example, the cross sections for hard electron-proton or electron-proton-electron-proton collisions that transfer large momentum can be expressed as a product of a short-distance partonic (quark or gluon) cross section, calculable in perturbative QCD, and parton distribution functions that encode the long-distance information on the structure of the proton.
Traditionally, the spatial view of nucleon structure provided by lattice QCD and the momentum-based view provided by the parton picture stood in stark contrast. Recent theoretical developments have clarified the connection between these two views, that of momentum and that of spatial co-ordinates, such that ultimately it should be possible to provide a complete “space-momentum” map of the proton’s internal landscape. These maps, referred to as Generalized Parton Distribu-tions (GPDs), describe how the spatial shape of a nucleon changes when probing different ranges of quark momentum. Projected along one dimension, GPDs reproduce the form factors; along another dimension they provide a momentum distribution. With enough information about the cor-relation between space and momentum distributions, one can construct a full “tomographic” image of the proton. The weighted integrals, or moments, of the GPDs contain information about the forces acting on the quarks bound inside the nucleon. These moments can now be computed using lattice QCD methods. Not only do they have an attractive physical interpretation, but they can also be compared directly to unambiguous predictions from the underlying theory. Higher-order QCD corrections for many observables sensitive to GPDs have now also been calculated, strengthening the theoretical underpinning for extraction of GPDs.
The simplest and cleanest experimental process that gives access to GPDs is Deeply Virtual Compton Scattering (or DVCS), where a virtual photon of high 4-momentum Q2 scatters off a nucleon producing a real photon on the final state (γ∗p→γp). Experimentally, DVCS occurs in the electroproduction of photons off the nucleon: ep→ epγ, where there is a competing channel, the Bethe-Heitler (BH) process, where the photon of the final state is radiated by the electron and not the proton. The BH process is, however, calculable in Quantum Electrodynamics given our knowledge of the proton form factors.
Extracting GPDs from DVCS requires the fundamental demonstration that DVCS is well de-scribed by the leading twist mechanism at finite Q2. This leading twist mechanism consists on the scattering on a single quark (or gluon) inside the nucleon.
2. Current experimental situation
The H1 [1, 2] and ZEUS [3] collaborations measured the cross section for xB ≈ 10−3. The
HERMES collaboration measured relative beam-helicity [4] and beam-charge asymmetries [5, 6]. Relative beam-helicity [7] and longitudinal target [8] asymmetries were measured at the Thomas Jefferson National Accelerator Facility (JLab) by the CLAS collaboration. The first dedicated
DVCS experiment ran in Hall A at JLab [9]. Helicity-correlated cross section of electroproduction of photons were measured with high statistical accuracy as a function of Q2(Fig. 1).
The combination of Compton form factors (CFF) extracted from these data (Fig. 2) was found to be independent of Q2, within uncertainties. This exciting result strongly supports the twist-2
dominance of the imaginary part of the DVCS amplitude. GPDs can then be accessed experimen-tally at the kinematics of JLab.
A dedicated DVCS experiment on the neutron (E03-106) followed E00-110. Using a deu-terium target, E03-106 provided the first measurements of DVCS on the neutron, particularly sen-sitive to the least known GPD E, and DVCS on the deuteron [10]. A first constraint on the orbital angular momentum of quarks inside the nucleon (Fig. 3) was obtained relying on the VGG model of GPDs [11].
3. Future DVCS program with CEBAF at 6 GeV
A new experiment E07-007 was recently approved with the highest scientific rating (A) by the JLab Program Advisory Committee (PAC-31) and will run later this year. E07-007 will use the important lessons we learned in E00-110 to further test and explore the potential of DVCS to measure GPDs.
In E00-110 we managed to determine the photon electroproduction helicity-independent (un-polarized) cross section. This additional information was not in the initial goal of the E00-110 experiment, and we were only able to measure it at one single value of Q2= 2.3 GeV2 (Fig. 1).
We found that the total cross section was much larger than the BH contribution. The excess in the cross section is then coming from both the interference term (BH–DVCS) and the DVCS2 contri-bution. These contributions contain different kinds of GPD combinations. The interference term is proportional to a linear combination of GPD integrals, whereas the DVCS2is related to a bilinear combination of GPD integrals.
The harmonicφ structure of the DVCS cross section does not allow the independent determi-nation of the BH-DVCS interference and the DVCS2contributions.
3.1 Interference and DVCS2separation
E07-007 was approved to measure the photon electroproduction helicity-independent cross section as a function of Q2 and at different incident beam energies. The different beam energy dependence of the DVCS2and BH·DVCS interference term complement the azimuthal analysis of
the cross section in order to provide a complete separation of these two contribution.
3.2 Separation of the longitudinalπ0electroproduction cross section
In addition to these fundamental measurements on DVCS, E07-007 will concurrently measure theπ0electroproduction cross section for different beam energies. We successfully measured the
π0 electroproduction cross section in E00-110. However, since we only ran at one incident beam
Deep exclusive reactions at Jefferson Lab Hall A Carlos Munoz Camacho
)
4
DVCS cross section (nb/GeV
2 = 1.9 GeV 2 Q 2 = 1.5 GeV 2 Q (deg) φ -0.02 0 0.02 90 180 270 (deg) φ -0.02 0 0.02 90 180 270 (deg) φ -0.02 0 0.02 90 180 270 (deg) φ -0.02 0 0.02 90 180 270 -0.04 -0.02 0 0.02 0.04 E00-110 Total fit Twist - 2 -0.04 -0.02 0 0.02 0.04 -0.04 -0.02 0 0.02 0.04 -0.04 -0.02 0 0.02 0.04 -0.02 0 0.02 -0.03 0 0.03 2
< t > = - 0.33 GeV < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
-0.02 0 0.02 -0.03 0 0.03 2
< t > = - 0.33 GeV < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
-0.02 0 0.02 -0.03 0 0.03 2
< t > = - 0.33 GeV < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
-0.02 0 0.02 -0.03 0 0.03 2
< t > = - 0.33 GeV < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
)
4
DVCS cross section (nb/GeV
beam helicity-independent beam helicity-dependent (deg) φ 90 180 270 E00-110 2 |BH+DVCS| 2 |BH| 2 - |BH| 2 |BH+DVCS| (deg) φ 90 180 270 (deg) φ 90 180 270 (deg) φ 90 180 270 E00-110 Total fit Twist - 2 0 0.05 0.1 -0.02 0 0.02 2
< t > = - 0.33 GeV 2 < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
< t > = - 0.33 GeV 2 < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
< t > = - 0.33 GeV 2 < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
< t > = - 0.33 GeV < t > = - 0.28 GeV 2 < t > = - 0.23 GeV 2 < t > = - 0.17 GeV 2
Figure 1: Data and fit to the helicity-dependent d4Σ/dQ2dx
Bdtdφ, and the helicity-independent
d4σ/[dQ2dx
Bdtdφ] cross sections, as a function of the azimuthal angleφbetween the leptonic and hadronic
planes. All bins are athxBi = 0.36. Error bars show statistical uncertainties. Solid lines show total fits with
one-σstatistical error bands. The green line is the|BH|2contribution to d4σ. The first three rows show the
helicity-dependent cross section for values of Q2= 1.5, 1.9 and 2.3 GeV2, respectively. The last row shows
the helicity-independent cross section for Q2= 2.3 GeV2.
) 2 -t (GeV 0.15 0.2 0.25 0.3 0.35 , E) H~ (H, I Im C 0 1 2 3 4 5 2 = 1.5 GeV 2 Q 2 = 1.9 GeV 2 Q 2 = 2.3 GeV 2 Q VGG model
Figure 2: Extracted imaginary part of the twist-2 angular harmonic as function of t. Superposed points are
offset for visual clarity. Their error bars show statistical uncertainties.
u J -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 d J -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 HERMES Preliminary p-DVCS JLab Hall A n-DVCS GPDs from :
Goeke et al., Prog. Part. Nucl. Phys. 47 (2001), 401. Code VGG (Vanderhaeghen, Guichon and Guidal)
0.14 ± = 0.18 5.0 u J + d J AHLT GPDs [36] Lattice QCDSF (quenched) [40] Lattice QCDSF (unquenched) [41] LHPC Lattice (connected terms) [42]
Figure 3: Experimental constraint on Ju and Jd quark angular momenta from the present n-DVCS
re-sults [10]. A similar constraint from the p-DVCS target spin asymmetry measured by HERMES, and differ-ent lattice QCD based calculations are also shown.
element to make a flavor decomposition of GPDs, which is impossible to obtain from DVCS on the proton alone.
E07-007 will test the Q2−dependence of the longitudinal cross section and check if the
factor-ization is applicable at the Q2 values currently accessible experimentally (around Q2∼ 2 GeV2).
If this test turns out positive, very interesting and complementary information on GPDs will be available from this channel.
3.3 E08-025: Rosenbluth-like separation on the neutron
Deep exclusive reactions at Jefferson Lab Hall A Carlos Munoz Camacho Bj x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ) 2 (GeV 2 Q 0 5 10 2 <4 GeV 2 W 11 GeV ≤ beam Unphysical with E = 6.6 GeV beam E = 8.8 GeV beam E = 11.0 GeV beam E = 5.75 GeV beam E
DVCS measurements in Hall A/JLab
Figure 4: Approved DVCS kinematics for H(e, e′γ)p measurements in Hall A with 3, 4, and 5 pass beams of CEBAF at 12 GeV. The diamond shapes trace the approximate acceptance of the Hall A High Resolution Spectrometer in each setting. The boundary of the unphysical region corresponds to the maximum possible Q2at a given xBfor 11 GeV. This corresponds to 180◦electron scattering, equivalent toθq= 0◦. The points
at EBeam= 5.75 GeV were obtained in E00-110.
4. 12 GeV program
The experimental GPD program is at the heart of the scientific motivation of the major Jef-ferson Lab upgrade to 12 GeV. E12-06-114 [12] is a wide experimental program of DVCS high precision cross-section measurements in Hall A with CEBAF at 12 GeV, enthusiastically approved by JLab PAC-30. With polarized 6.6, 8.8, and 11 GeV beams incident on the liquid hydrogen tar-get, we will provide a major survey of proton DVCS in almost the entire kinematic range accessible with CEBAF at 12 GeV (Fig. 4).
References
[1] A. Aktas et al., Eur. Phys. J. C44, 1 (2005). [2] C. Adloff et al., Phys. Lett. B 517, 47 (2001). [3] S. Chekanov et al., Phys. Lett. B573, 46 (2003). [4] A. Airapetian et al., Phys. Rev. Lett. 87, 182001 (2001). [5] F. Ellinghaus, Nucl. Phys. A711, 171 (2002).
[6] A. Airapetian et al., (2006).
[7] S. Stepanyan et al., Phys. Rev. Lett. 87, 182002 (2001). [8] S. Chen et al., Phys. Rev. Lett. 97, 072002 (2006).
[9] C. Muñoz Camacho et al., Phys. Rev. Lett. 97, 262002 (2006). [10] M. Mazouz et al., Phys. Rev. Lett. 99, 242501 (2007).
[11] M. Vanderhaeghen, P. A. M. Guichon, and M. Guidal, Phys. Rev. D60, 094017 (1999). [12] J. Roche et al., JLab proposal PR12-06-114 (2006).
